GEOCHEMISTRY AND HYDRAULIC FRAMEWORK OF AN ISOLATED HOT SPRING, TECOPA BASIN, CALIFORNIA

Tecopa Hot Springs is an isolated hot spring discharge area located in the Tecopa Basin, southeastern California. Isolated hot springs (e.g., those not related to an obvious regional thermal feature) are uncommon and the hydrologic processes behind their occurrence are poorly understood. In this study, springs and seeps in the Tecopa Hot Springs area were sampled and analyzed for their chemical and isotopic composition. The spring analysis along with detailed geologic mapping in the Tecopa Hills, where the springs discharge, were conducted to better understand the hydrologic processes in the discharge area and source of heat for the springs.

Seven springs in the Tecopa Hot Springs area and two other nearby springs were sampled in May 2006 for major, minor, and trace element chemistry, and oxygen and hydrogen stable isotopes. In addition, thermal water from Grimshaw spring (Tecopa Hot Springs) and ambient-temperature waters from Shoshone and Amargosa Borax Works springs were sampled for noble gas and Sr isotope compositions. The water chemistry compositions at Tecopa Hot Springs generally fall into three categories: (1) pH-neutral to slightly alkaline, Na+Ca+Mg+SO₄+HCO₃ water with moderate dissolved solids, (2) slightly alkaline, Na+Cl+SO₄+HCO₃ thermal water with moderate to high dissolved solids, and (3) alkaline Na+Cl+SO₄+HCO₃ waters with high dissolved solids. Whereas the first two categories fall into regionally defined water compositions (Larsen et al, 2001), category 3 waters appear to be partially evaporated runoff from the thermal springs. Noble gas-based recharge temperatures, corrected for excess air, range from 15 to 25 degrees Celsius, suggesting external thermal input. However, only Grimshaw spring shows a significant mantle or crustal gas component. R/Ra values for Grimshaw, Shoshone, and Amargosa Borax Works are 0.08, 0.10, and 0.13, respectively. Given these values, the waters are interpreted to have accumulated ⁴He along long flow paths with Grimshaw waters receiving the greatest crustal flux of heat, ⁴He and other noble gases. Geologic mapping suggests that spring discharge is related to ground-water flow from the northeast, with water moving along a subdominant northeast-oriented joint system.